1. Field of the Invention
The invention relates generally to systems for making correlation measurements using broadcast spread-spectrum signals and, in particular, to systems that make correlation measurements using pulse shape measurements.
2. Background Information
One example of a system that utilizes broadcast spread-spectrum signals is a global positioning system, such as, GPS, GLONASS or GALILEO systems. The system receivers determine their global positions based on the signals they receive from associated satellites. The broadcast spread-spectrum signal consists of a carrier that is modulated by at least one pseudorandom code, such as a binary PRN code that consists of a pseudo-random sequence of ones and zeros that periodically repeat. The ones and zeros in the PRN code are referred to as “code chips,” and the transitions in the code from one to zero or zero to one, which occur at “code chip times,” are referred to as “chip transitions.” Each satellite uses a unique PRN code, and thus, a receiver can associate a received signal with a particular satellite by determining which PRN code is included in the signal.
The receiver calculates the difference between the time a satellite transmits its signal and the time that the receiver receives the signal. The receiver then calculates its distance, or “pseudo-range,” from the satellite based on the associated time difference. Using the pseudo-ranges from at least four satellites, the receiver determines its global position.
To determine the time difference, the receiver synchronizes locally-generated PRN codes with the PRN codes in the received signal by aligning the code chips in respective local codes with the chips in the corresponding satellite generated PRN codes. It then determines how much the locally-generated PRN codes are shifted, in time, from the known timing of the satellite PRN codes at the time of transmission, and calculates the associated pseudoranges essentially by multiplying the measured time shifts by the speed of light. The more closely the receiver aligns the locally-generated PRN code with the PRN code in the received signal, the more precisely the receiver can determine the associated time difference and pseudorange and, in turn, its global position.
The code synchronization operations include acquisition of the satellite PRN code and tracking the code. To acquire the PRN code, the receiver generally makes a series of correlation measurements that are separated in time by, for example, a code chip, to determine when the locally-generated code aligns with the received code to within one code chip. To thereafter track the PRN code, the receiver generally makes correlation measurements that are associated with the received PRN code and early and late versions of the locally-generated PRN code. Using the early and late correlation measurements, the receiver produces an associated error signal that is proportional to the misalignment between the local PRN code and the received PRN code. The error signal is used, in turn, to control the clocking speed of the local PRN code generator, which essentially shifts the local PRN code to minimize the error signal.
The receiver also typically aligns the satellite carrier with a local carrier using correlation measurements associated with a punctual version of the local PRN code. To do this the receiver uses a carrier tracking phase lock loop.
The receiver receives not only line-of-sight, or direct path, satellite signals but also multipath signals, which are signals that travel along different paths and are reflected to the receiver from the ground, bodies of water, nearby buildings, etc. The multipath signals arrive at the receiver after the direct-path signal and combine with the direct-path signal to produce a distorted received signal. This distortion of the received signal adversely affects code synchronization operations because the correlation measurements, which measure the correlation between the local PRN code and the received signal, are based on the entire received signal—including the multipath components thereof. The distortion may be such that the receiver attempts to synchronize to a multipath signal instead of to the direct-path signal. One way to more accurately synchronize the received and the locally-generated PRN codes is to use the “narrow correlators” discussed in U.S. Pat. Nos. 5,101,416; 5,390,207 and 5,495,499. It has been determined that narrowing the delay spacing between early and late correlation measurements substantially reduces the adverse effects of noise and multipath signal distortion on the early-minus-late measurements. The narrow correlators are essentially spaced closer to a correlation peak that is associated with the punctual PRN code correlation measurements than the contributions of many of the multipath signals. Accordingly, the early-minus-late correlation measurements made by these correlators are significantly less distorted than they would be if they were made at a greater interval around the peak.
Another correlation technique makes non-zero correlation measurements near chip transitions in the locally-generated PRN code and zero valued correlation measurements otherwise. This technique, which is referred to herein as “blanked correlation,” is described in U.S. Pat. No. 6,243,409.
Yet another way to more accurately synchronize the received and the locally-generated PRN codes is to use a multipath mitigation processing technique that iteratively produces estimates of the direct path signal and one or more of the multipath signals. One such technique, which uses multiple correlators arranged on either side of the correlation peak is described in U.S. Pat. Nos. 5,615,232 and 5,692,008. Another technique that uses multiple correlators is described in U.S. Pat. No. 5,414,729. Yet another multipath mitigation technique is discussed in Weill, “Multipath Mitigation Using Modernized GPS Signals: How Good Can It Get,” ION GPS 2002, Portland, Oreg., Sep. 24-27, 2002.
The receivers and, in particular, the correlation hardware used for signal acquisition and tracking are constructed to operate with selected correlation and multipath mitigation schemes. Thus, a receiver that operates with conventional parallel early, punctual and late correlators positioned about the correlation peak includes the three correlators and associated circuitry that provides the corresponding early, punctual and late versions of the local PRN code. If the receiver instead uses multiple early and late correlators, the receiver includes a sufficient number of the correlators and the circuitry required to produce the associated versions of the local code. If a different correlation and/or multipath mitigation technique is to be used, the receiver hardware must typically be re-designed to include the necessary correlators and/or the circuitry required to produce the corresponding local versions of the PRN code.
Also, a receiver using the narrow correlation technique discussed above may require an additional set of more widely spaced correlators for use in re-acquiring the GPS code if the narrowly spaced correlators should lose lock during tracking. Thus, such a system requires two sets of early and late correlators as well as the associated circuitry to produce the corresponding versions of the GPS code.